A radar level gauge (RLG) is suitably used for making measurements of a filling level of products such as process fluids, granular compounds and other materials contained in a tank.
An example of such a radar level gauge can include transceiver circuitry for transmitting and receiving microwaves, a signal propagation device arranged to direct microwaves towards the surface and to return microwaves reflected by the surface to the transceiver, and processing circuitry adapted to determine the filling level based on a relation between microwaves transmitted and received by the transceiver.
The signal propagation device may include a wave guiding structure, such as one or several hollow waveguides and a directional antenna, adapted to emit free propagating electromagnetic waves into the tank and receive a reflection of those waves. Such an RLG is sometimes referred to as a non-contact RLG. The antenna may be adapted to a specific frequency band. The currently most used frequency bands have center frequencies of around 6 GHz or 24 GHz, although higher frequency bands may also be contemplated.
In one conventional design the wave guide structure comprises a hollow wave guide which extends through the tank wall, and connects the directional antenna to the transceiver circuitry. A dielectric filling member is sometimes arranged in the cavity of the antenna and/or the wave guide, in order to protect the waveguide/antenna from thermal and chemical influence from the tank atmosphere. The dielectric filling member should be transmissive to microwaves, chemically resistant, preferably hydrophobic (repelling to water), and also needs to withstand temperatures and pressure which are present in the tank. Some applications, known as high-pressure, high temperature (HPHT) applications, present especially challenging conditions. One material which is often used for such filling members is PTFE, also known as Teflon®.
One or several sealing elements, such as O-rings or the like, are typically arranged in groove(s) formed in the plug to provide a process seal ensuring that the contents of the tank are not released into the outside environment. The process seal may be pressure tight. An O-ring, or possibly another type of annular member, may also have the function of mechanically fixating the filling member in the cavity.
There are a few challenges with such dielectric filing members. One potential problem is that suitable materials such as PTFE have large coefficients of thermal expansion, and will expand at elevated temperatures. This may cause stress, deformation and fatigue in the filling member, and eventually fractures. A solution to this problem is to make the filling member slightly smaller than the antenna cavity, so as to leave room for thermal expansion. In order to avoid that the gap formed between the filling member and the inside of the antenna causes interference with the microwave signals, a sleeve of microwave absorbing material may be arranged around the filling member. This is disclosed e.g. in U.S. Pat. No. 7,864,104, where an attenuating foil is fixed to a dielectric filling body with an adhesive film.
Another challenge is that the filing member may cause resonances in the microwave signals. In order to eliminate, or at least reduce, such resonance, it is known to embed an microwave absorbing pin in the filing member. The pin may be made of metal or Eccosorb®, and should be completely concealed in the filling member material so as to not be subject to the tank atmosphere. For example, it is known to insert the pin in a hole extending from the outer face of the filling member, and then glue a PTFE plug in the hole to seal it.
The combination of a dielectric filling body, made e.g. of PTFE, with one or several microwave absorbing elements may be difficult to manufacture in a reliable manner, without introducing additional interfaces, such as adhesive films or glue.
One specific challenge is that some polymers, such as PTFE, are not liquid when melted. This means that such materials cannot be injection molded, and also cannot be welded or otherwise joined by melting.